This invention relates to methods of forming optical conversion layers for semiconductor light emitting devices and devices so formed.
Light emitting diodes (LEDs), a particular type of light emitting device, are semiconductor devices that convert electric energy to light. Inorganic LEDs typically include an active layer of semiconductor material and a P-N junction formed at an interface between two oppositely doped layers. When a bias is applied across the P-N junction, holes and/or electrons are injected by the P-N junction into the active layer. Recombination of holes and electrons in the active layer generates light that can be emitted from the LED. The structure of the device, and the material from which it is constructed, can determine the intensity and wavelength of light emitted by the device.
Light from a blue emitting LED has been converted to white light by surrounding the LED with a yellow phosphor, polymer or dye, such as cerium-doped yttrium aluminum garnet (Ce:YAG). The phosphor material can absorb and “downconvert” some of the blue light generated by the LED. That is, the phosphor material generates light, such as yellow light, in response to absorbing the blue light. Thus, some of the blue light generated by the LED appears to be converted to yellow light. Some of the blue light from the LED, however, passes through the phosphor without being converted. The overall LED/phosphor structure can emit both blue and yellow light, which combine to provide light that may be perceived as white light.
LEDs have been combined with phosphor layers by dispensing a volume of phosphor-containing encapsulant material (e.g., epoxy resin or silicone) to cover the LED.
Another conventional method for coating an LED is by stencil printing. In a stencil printing approach, multiple LEDs are arranged on a substrate with a desired distance there between. A stencil is positioned on the substrate with each of the LEDs located within a respective opening in the stencil. A composition is then deposited in the stencil openings, covering the LEDs, with a typical composition being a phosphor in a silicone polymer that can be cured by heat or light. After the holes are filled, the stencil is removed from the substrate and the stenciling composition is cured to a solid state.
Another conventional method for coating LEDs with a phosphor utilizes electrophoretic deposition (EPD). The conversion material particles are suspended in an electrolyte based solution. A plurality of LEDs are immersed in the electrolyte solution. One electrode from a power source is coupled to the LEDs, and the other electrode is arranged in the electrolyte solution. The bias from the power source is applied across the electrodes, which causes current to pass through the solution to the LEDs. This creates an electric field that causes the conversion material to be drawn to the LEDs, covering the LEDs with the conversion material. After the LEDs are covered by the conversion material, they are removed from the electrolyte solution so that the LEDs and their conversion material can be covered by a protective resin.
Still another coating method for LEDs utilizes droplet deposition using systems similar to those in an ink-jet printing apparatus. Droplets of a liquid phosphor-containing material are sprayed from a print head. The phosphor-containing droplets are ejected from a nozzle on the print head in response to pressure generated in the print head by a thermal bubble and/or by piezoelectric crystal vibrations.